Thanks, However, my bank statements don't show times. The PM coffee I took on the date in question is there on one CC (but the date only although it would have been about 15:30) and I know from some other records I bought a Sainsbury's "meal deal" there for lunch, almost certainly for cash. I can't (yet) find any evidence for CC purchases in between. And I don't have tracking enabled on anything I can think of.
However the point is really for them to prove I didn't exit and enter again rather than the other way round. Their photo evidence does not do that should this go to any proper forum.
The Sainsbury's store under-manager was very unhelpful. Similar from the Sainsbury's Careline. I do see the real store manager in the café regularly. I might see if he recognizes me and will be more useful.
I see from online research that "double dipping" speculative invoices (not fines) are quite common from ANPR-controlled car parks. I will read further about the successes reported and see what can be done. I have a draft letter to send. The £40 sum is not a big deal but I do very regularly visit the store twice a day.